This study will determine if the local regulation of the bronchial circulation is mediated by mechanisms similar to those controlling the pulmonary circulation or other systematic beds. While the bronchial circulation is a systemic bed, it is in close proximity to the pulmonary circulation and both beds may be similarly controlled. Support of tracheobronchial functions would best be served by sysmetic types of control, but in diseases where there is a deficiency of pulmonary blood flow (congenital lack of the pulmonary artery), gas exchange would best be served by pulmonary types of control. Experiments will be conducted on chloralose anesthetized, thoracotomized sheep. The local response to hypoxia will be determined by ventilating the left lung with a hypoxic gas while the right lung is ventilated with 100 percent O2 to prevent systemic hypoxia. Bronchial blood flow to the left lung with be measured with an electromagnetic flowmeter on the carinal artery. The right branch of this artery will be ligated. A vasodilation will indicate that a systemic mechanism is operative; a vasoconstriction will indicate the pulmonary-hypoxic-vasoconstrictor mechanism. The pulmonary vascular response of the left lung will be observed for comparison. In the systemic circulation, an increase in PCO2 causes a vasodilation, but causes a vasoconstriction in the pulmonary circulation. The bronchial vascular response to unilateral ventilation with various CO2 levels will also be determined. A hypocapnic vasodilation may be due to a pulmonary type of regulation. Alternatively, bronchial vasodilation may be due to a functional hyperemia (a systemic response) secondary to hypocapnic bronchoconstriction. To distinguish between these two possibilities, pulmonary resistance and compliance of the left lung will be determined and correlated with the vascular response. The pulmonary and bronchial vascular responses to perfusion of the bronchial circulation with blood tonometered with various O2 and CO2 gases will also be determined. The systemic circulation autoregulates blood flow over a range of perfusion pressures, but the pulmonary circulation does not. Hence, the changes in bronchial blood flow in response to variations in perfusion pressure produced by inflating balloons in the aorta above and below the bronchial arterial source during pharmacologic blockade of the autonomic nervous system will be examined.